We have identified and purified a new DNA glycosylase from e. coli cell extracts. This enzyme, also found in rodent liver, is specific for excising alkylated guanines with opened imidazole rings from DNA. These ring opened guanines are called formamido-pyrimidine (FAPy) and the enzyme is called FAPy-DNA glycosylase. The substrate we use in standard reactions is prepared by methylating DNA with dimethylsulfate followed by treating the DNA with 0.2N NaOH to induce imidazole ring opening. Othe laboratories have now shown that there is spontaneous ring fission in guanines alkylated with phosphoramide mustard at pH 7.4, 37 C, and we have shown that such adducts are excised from DNA by FAPy-DNA glycosylase. We also have preliminary results showing that the spontaneously ring opened aflatoxin B1-N7 guanine addunts are removed from DNA by FAPy-DNA glycosylase. We have found the presence of aflatoxin V1 in rat liver to also induce an increase in enzyme levels. We propose to study the excision-repair of these ring opened guanine adducts in greater detail. The experiments will include those in which we inject Fischer rats with [3H] aflatoxin B1, and extract liver DNA from rats sacrificed after varying time periods of injection. We shall on the one hand try to follow the fate of aflatoxin B1 guanine adducts in such DNA and on the other hand measure the aflatoxin B1, aflatoxin B1-N7-guanine, and aflatoxin B1-FAPy pools in the cell extract fraction that is 60% methanol soluble. This regime of experiments is designed to determine whether all the aflatoxin B1-N7-guanine adducts in DNA are converted to the ring opened derivatives before their removal. We plan to treat rats with aflatoxin B1 for varying time periods and then measure enxyme levels to determine if the rise in enzyme levels coincides with the increase in the amounts of aflatoxin B1-FAPy residues in cellular DNA. We further plan to determine whether other polynuclear aromatic hydrocarbons that induce ring opening in guanines, have the effect of subjecting such ring opened adducts to excission by FAPy-DNA glycosylase.